Leaf spring construction

ABSTRACT

An improved leaf spring construction adapted to be suspended at opposite ends thereof and loaded at its mid-portion characterized in that said construction comprises at least one leaf element made from a flat sheet of uniform width and thickness with the intermediate portion of the spring including said mid-portion and extending over 50 percent of the length of said spring. The spring construction further having an arculate cross-section with its convex side directed upwardly, the curvature of the arc diminishing toward and into flat end portions of the spring. The form of the cross-section of the leaf of said spring construction being additionally characterized in that the angle of the sector of a circle at the intermediate portion and which intersects the thicknesswise bisector line of the arc at the edges of the leaf and the widthwise bisector point of the line covering the line between the edges of the leaf spring is not more than 210*.

United States Patent [191 Fukui et al.

June 4, 1974 1 LEAF SPRING CONSTRUCTION [75] Inventors: Hiroshi Fukui, Gifu-ken; Gakuji Iwatsu, Tokai; Junichi Kato, Nagoya, all of Japan [73] Assignee: Aichi Steel Works, Limited,

Aichi-ken, Japan [22] Filed: Sept. 26, 1972 [21] Appl. No.2 292,298

Related U.S. Application Data [63] Continuation-impart of Scr. No. 104,018, Jan. 5,

1971, Pat. No. 3,705,718.

{30] Foreign Application Priority Data Dec. 20, 1971 Japan 46-103416 [52] U.S. Cl. 267/47 [51] Int. Cl Fl6f H22 [58] Field of Search 267/47 [5 6] References Cited UNITED STATES PATENTS 3,281,139 10/1966 Faherty, Jr 267/47 3,534,951 10/1970 Brownyer 267/47 Primary Examiner-James B. Marbert Attorney, Agent, or Firm-Karl W. Flocks [57] ABSTRACT An improved leaf spring construction adapted to be suspended at opposite ends thereof and loaded at its mid-portion characterized in that said construction comprises at least one leaf element made from a flat sheet of uniform width and thickness with the intermediate portion of the spring including said mid-portion and extending over 50 percent of the length of said spring. The spring construction further having an arculate cross-section with its convex side directed upwardly, the curvature of the arc diminishing toward and into flat end portions of the spring. The form of the cross-section of the leaf of said spring construction being additionally characterized in that the angle of the sector of a circle at the intermediate portion and which intersects the thicknesswise bisector line of the arc at the edges of the leaf and the widthwise bisector point of the line covering the line between the edges of the leaf spring is not more than 210.

4 Claims, 24 Drawing Figures PATENTEDJUN 4mm 3314.410

SHEET 1 OF 8 FIG. I

PATENTEDJUN 4 mm 38 14141 O SHE! a of 8 FIGS PATENTEDJUN 41914 3.?3141410 SHEET 5 0F 8 PATENTEDJUN 41914 SHEET 7 OF 8 FIG. 23

A nx mzxvN ZOFQMm m0 933002 ANGLE 0F ARC 6(1'Od) This application is a continuation-in-part of our copending application Ser. No. 104,018 filed Jan. 5, 1971, now U.S. Pat. No. 3,705,718.

BACKGROUND OF THE INVENTION Heretofore leaf springs have been commonly used in the form of a cantilever or semi-elliptic leaf spring. One known type leaf spring construction has varying thickness along its length and is made by laminating a plurality of spring elements of different lengths or by using a tapered spring element, so that the spring has a maximum thickness at a portion where a maximum bending moment is produced. Among the known types of leaf springs, a laminated spring has most commonly been employed; however, it is disadvantageous in that substantial friction is produced between leaf spring elements and that it is relatively heavy in weight. A tapered leaf spring has disadvantages in that it is difficult to manufacture and to obtain products of uniform quality, so that it is not widely used.

Recently, the use of a leaf spring comprising a single spring element having a channel-shaped cross-section of uniform thickness has been proposed. However, since the characteristics of the spring of this type are not fully known yet, it is not actually used.

SUMMARY OF THE INVENTION The present invention relates to a leaf spring and has an object to provide a light weight leaf spring which can be readily designed and manufactured.

Another object of the present invention is to provide a leaf spring which has an increased fatigue life and uniform stress distribution characteristics.

The inventors have found that, in a leaf spring comprising a single leaf spring element which has an arcuate cross-section of a uniform thickness, it is possible to obtain a uniform stress distribution by making the radius of curvature of the arc of the cross-section minimum at a point where the bending moment is largest while gradually increasing the radius towards an end apart from the said point of maximum bending moment. They also found that the thickness and the width of the spring element determine the minimum section modulus and both ends of the spring may have flat cross-section.

Thus, according to the present invention, there is provided a leaf spring adapted to be suspended at both ends of the spring and loaded at the mid-portion thereof, said spring comprising at least one leaf element made of a flat sheet of uniform width and thickness, the intermediate portion of the spring including the midportion and extending over 50 percent of the length of the spring having arcuate cross-section with its convex side directed upwardly, the curvature of the arc diminishing towards and into flat end portions of the spring, the form of the crosssection of the leaf at the intermediate portion being such that the angle of the sector of a circle, which intersects the thicknesswise bisector line of the arc at the edges of the leaf and the widthwise bisector point of the line, covering the line between the edges of the leaf being not more than 210. When the feature of the present invention is embodied in a semielliptic leaf spring, eyelet portions can readily be formed.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the advantages of the present invention can be more clearly recognized, the invention will now be described taking reference to the accompanying drawings, in which:

FIG. 1 is an enlarged cross-sectional view of a blank which is used to produce a leaf spring of the present invention;

FIG. 2 is a side view showing a leaf spring embodying the present invention;

FIG. 3 is a plan view thereof;

FIGS. 4 and 5 are enlarged sectional views taken along the lines IV-IV and VV in FIG. 3, respectively;

FIG. 6 is a side view with a portion broken away of a vehicle suspension mechanism using the leaf spring;

FIG. 7 is an enlarged fragmentary sectional view thereof;

FIG. 8 is a perspective view of an intermediate member used in the suspension mechanism;

FIG. 9 is a plan view of the second embodiment of the present invention;

FIG. 10 is a side view thereof;

FIG. 11 is a side view of the third embodiment of the present invention;

FIG. 12 is a plan view thereof;

FIG. 13 is an enlarged sectional view taken along the line XIII-XIII in FIG. 12;

FIG, 14 is an enlarged fragmentary sectional view of a vehicle suspension mechanism using the leaf spring of the third embodiment;

FIG. 15 is a perspective view of an intermediate member used in the mechanism;

FIG. 16 is a sectional view similar to FIG. 13 but showing a modification of the embodiment;

FIG. 17 is an enlarged fragmentary sectional view of a vehicle suspension mechanism using the leaf spring shown in FIG. 16;

FIG. 18 shows an intermediate member used in the mechanism;

FIGS. 19 and 20 show cross-sectional enlarged views I of further embodiments of the present invention;

FIG. 21 is a front view, with portions broken away, of a vehicle suspension mechanism using the leaf spring of FIG. 20;

FIGS. 22 and 24 are diagrams showing the relationship between the including angle of part circular crosssection of the leaf spring of the present invention and the ratio of the width to thickness of the spring blank, the ratio of the length of flat portion to the spring length being taken as a parameter; and

FIG. 23 is a diagram showing the relationship between said angle and the section modulus of leaf spring.

PRINCIPLES AND EMBODIMENTS OF THE INVENTION A blank of leaf element having a width b and a thickness h as shown in FIG. 1 is used to form a semi-elliptic longitudinally symmetric leaf spring 1 as shown in FIGS. 2 and 3. The spring 1 has acurved portion 2 with an arcuate cross-section, the radius of curvature being minimum at the mid-portion while gradually increasing towards the opposite ends of the spring. The spring 1 also has eyelet portions 3 at the opposite ends thereof.

Flat portions 4 are formed between the eyelet portions 3 and the curved portions 2.

In order to ascertain the advantageous feature of the present invention, the inventors have performed the following calculations.

In the calculation, the load and the allowable maximum stress have been determined assuming that the spring is used in a rear axle suspension mechanism of a car A which is a motorcar having a gross weight of l. 1 tons, a car B which is a motorcar having a gross weight of 1.2 tons, a car C which is a 4-ton truck, a car D which is an 8-ton truck and a car E which is an 1 1.5-ton truck. As shown in FIGS. 1, 4 and 5, the curved portion of the leaf spring has a cross-section of pure arcuate shape with the angle O'between the lines A-D and B-D equal to three radians (approximately 180) at the loading point and gradually decreasing to zero at the conjunction of the curved portion 2 and the flat portion ,4, where the points A and B are intersections between the transverse center line of each cross-section of the blank and the opposite side surfaces, and the point D is the center of the circle of the arcuate section. In the drawings, the reference character C designates the cen-' ter of the transverse center line.

In this geometrical configuration, the section modulus Z can be represented by the following equation:

Z=kb

where, K =f(0, a), a b/h.

In a simple beam which is subjected to a bending moment, the relationship between the bending moment Mx at a point apart by a distance x from a support of e b am. the m dtt ys. lmfssctiqaatthe aheysmsn;

tioned point and the stress can be represented by the following equation:

Zx Mx/o' Further, representing the effective length of the spring between the eyelet portions 3 by I, the distance from one of the eyelets 3 by x, and the load acting on the spring by P, the bending moment can be represented by the following equation:

Since x is equal to [/2 at the loaded point, the maximum bending moment can be represented as follows:

Mmax l/4 pl From the equations (1) through (3), it is possible to determine the maximum section modulus for the cars A through E with the same effective spring length. Thus, calculation was performed with respect to leaf spring blanks having the section ratio (a) of width to thickness (b/h) equal to 10 and 20. The results of the calculation made on blanks having the section ratio (a) of 10 are shown in Table 1 while the results on blanks having the section ratio (a) of 20 are shown in Table 2. From the Tables 1 and 2, it should be noted that. when the maximum value of the angle ADB is unchanged, the ratio of the length of the flat portion to TABLE 1 (a=l0) Type of Car Load P(kg) Spring Allowable Maximum Section Width(b) and I Flat Portion (R,)

length maximum bending modulus thicknessUt) of spring I( mm) stress moment Zmax (mm) a(kg/mm") Mmax .(mm)

(kg-mm) b It Z(mm-")' Length X Ratio to 24(mm) spring length A 245 1100 80 67375 842 52.0 5.2 234 152 27.8 B 290 1200 70 87000 1242 59.3 5.9 344 166 27.7 C 2610 1300 848250 16965 141.0 14.1 4672 I79 27.6 D 4795 1600 50 1923000 38460 186.0 18.6 10724 223 28.0 E 7473 1270 40 2372519 59313 215.0 21.5 16563 177 28.0

TABLE 2 a =20 Type of Width and thickness Flat portion (R car of spring (mm) h It Z(mm) Length X Ratio to 24(mm) spring length 6 1 A Load. 63.0 3.2 107 12.8 B spring length. allowable 71.7 3.6 154 74 12.4 C maximum stress tr, maximum 171.0 8.6 2107 12.4 D bending moment Mmax. and 225.0 1l. 3 4788 99 12.4 E section modulus Zmax are the 260.0 13.0 7323 78 12.4

same as in Table 1.

In view of the above facts, the inventors have performed claculations with respect to various radius of curvature at the loading point and various spring length and allowable maximum stress, and found that a substantially uniform stress distribution can be obtained under a constant ratio of the length of the flat portion to the effective spring length, irrespective of the change of the load, the spring length and the allowable maximum stress, provided that the ratio of the width to thickness of the spring element blank is the same. This can be theoretically explained as follows.

From the equations (1), (2) and (3), b can be represented by the following equation:

From the equations (4) and (5), it will be seen that the length (X of the flat portion, which amounts to x in case of Z; Zr, and the ratio (8) f the total length (.ZX,) to the spring length (I) must meet the following relations:

Thus, it should be noted that, in order to obtain a uniform stress distribution, the ratio of the length of the flat portion to the spring length must be determined in accordance with the ratio (a) of the width to thickness of the spring blank and the constant (K) but irrespective of the effective spring length, the allowable maximum stress 0' and the load (P).

In an arcuate cross-section, the constant (K) can be rsvreeatsdbzaths following ssa a It is apparent that, when the curved portion having an arcuate cross-section is so designed as to have the angle (0) exceeding 180 at or near the loading point or the center of the spring, press forming operation of the spring will become very difficult. Therefore, when the curved portion is so designed as to have an arcuate cross-section, the angle (0) must not exceed three radians (approximately 180). The relationship between the section modulus (Z) and the angle (0) is plotted in FIG. 23 in accordance with the equation (7) taking the ratio (a) of the width and thickness of the spring blank as a parameter. As seen from the drawing, each of the curves shows upwardly concave shape in the range of the angle (0) lower than 1.5 radian, in which range the section modulus is substantially greater than that of a flat leaf spring (6= 0). As the angle (0) exceeds 1.5 radian, the curve assumes an upwardly convex shape and the section modulus increases substantially constantly until the angle (6) increases to 3.0 radian. Thus, it is noted that the range of angle (0) from 1.5 to 3.0 radian 1? mrasts aven entraa s f r sprin de i n.

The configuration in that the angle (0) is equal to l.5

radian is substantially as shown in FIG. 5 and, if the angle (0) at the center of the spring is lower than this -YQIPEHQ ffestiterssultsan be btaine 7.

In F IG. 22, the point of intersection between the vertical line corresponding to the range (0) of L5 radian and the curve representing the value (8) which is equal to percent corresponds to the ratio (a) of 10, while that between the-vertical line of the angle (0) of 3.0 radian and the above curve corresponds to the ratio (a) of 5.9. The fact that the ratio (a) of the width and thickness of the leaf spring blank is equal to 5.9 means that the blank is relatively thick, so that a blank having the ratio (a) lower than 5.9 will be very difficult to be subjected to forming operation. Contrary, a blank hav ing the ratio (a) exceeding 30 is a very thin one for a leaf spring and provides an excessively weak resiliency. In FIG. 22, the point of intersection between the horizontal line corresponding to the ratio (a) of 30 and the vertical line corresponding to the angle (0) of 3.0 radian is on a curve corresponding to the value 8 of 9.1 B fill i From the above, it should be noted that, when the curved portion of the spring is so designed as to have noted that in the diagram of FIG. 22 each of the leaf springs shown in Table l is represented by the point R, while that shown in Table 2 is represented by the point R an arcuate cross-section, the ratio of the width to thickness of the blank must be within the range between 5.9 to 30 and the angle (0) at the loading point or the center of the spring must be within the range from to while the length of the flat portion of the spring must be within 9.1 to 50 percent of the effective spring length, in order to have a substantially uniform stress distribution. Examples of springs satisfying the above requirements are shown in Table 3. In FIG. 22, the points R through R, correspond to the Examples 1 TABLE 3 Example I (Rd) Example 2 (R Example 3 (R,,) Example 4 (R.,)

Load P (kg) 1,000 1,000 1.000 500 Spring Length 1 (mm) 1,200 1.200 1,200 1.200

Allowable Maximum Stress a 60 61) 75 60 (kg/m Spring Dimension bXI-i (mm) 100x10 Il 8 100x10 100 5 Angle at the Center Omax (deg) 137.5 136.3 106.1 120.0

Length of Flat Portion X 200 I40 250 100 (mm) Ratio of Flat Portion to Total 33 23 I 42 17 Length 6 (72) Length of Support Portion Is 0 0 0 0 (mm) Angle 0 Distance from the center of rad deg rad deg rad deg rad deg eyelet x (mm) 0 0 0 0 O 0 0 0 0 50 0 0 0 0 0 0 0 0 100 0 0 0 0 0 0 0 0 140 0 0 0 0 0 0 0.26 14.9 150 0 0 0.10 5.7 O 0 0.32 18. 200 0 0 0.58 33.2 0 0.61 35.0 250 0.90 51.6 0.93 53.3 0 0 0.88 50.4 300 1.15 67 1.10 63.1 0.78 44.7 1.08 61.8 350 1.35 77 1.38 79.2 1.07 61.4 1.23 70.6 400 1.52 87 1.55 88.9 1.25 71.7 1.41 80.5 450 1.72 99 1.75 100 1.40 80.2 1.57 90.0 500 1.93 111 1.95 111 1.53 87.6 1.73 99.2 550 2.17 I 124 2.18 125 1.70 97.4 1.91 109 600 2.40 138 2.38 136 1.85 106.1 2.10 v 120 A suspension mechanism employing the leaf spring as described above is illustrated in detail in FIGS. 6-8. In

FIG. 6, one of the eyelet portions 3 of the leaf spring 1 is received on a shaft 7'mounted on a bracket 6 which is projecting from a vehicle frame 5. The other eyelet portion 3 is supported on a shaft 10 provided on one end of a link 9 which is pivotally supported by a bracket 8 on the vehicle frame 5. At the mid-portion of the leaf spring 1, there is supported an axle 11 by means of U- bolts 12, a platemember I3 and nuts 14. In order to make the position of the axle 11 stabilized, an intermediate member l5may be disposed between the axle ll and leaf spring 1 as shown in FIGS. 7 and 8. The intermediate member 15 has a concave surface 16 of an arcuate cross-section at one side thereof so as to cooperate with the convex outer surface of the leaf spring 1, as well as a part-cylindrical concave surface 17 which extends perpendicularly to the concave surface 16 and cooperates with the outer surface of the axle ll. Preferably, the leaf spring 1 is provided with a through-hole or recess 18 which cooperates with a projection 19 formed on the concave surface 16 of the intermediate member 15. This arrangement will be effective to securely hold the axle 11 on the leaf spring 1. Alternatively, the projection may be formed on the leaf spring 1 and the recess or through-hole on the concave surface 16 of the intermediatemember 15.

FIGS. 9 and 10 show a modification of the spring shown in FIGS. 2 and 3, in which the width of the flat portion 4 of the spring 1 is gradually reduced toward the eyelet portion 3. In other words, the section modulus (2) at the flat portion 4 is also changedin accordance with the distance from the center of the eyelet portion 3. This design is effective to obtain a more uniform stress distribution throughout the effective length of the leaf spring. I FIGS. 11-15 show anotherembodiment of the leaf spring which has a supporting portion 20 having the same cross-section at the center thereof through the length (Is). This design is effective to securely mount the axle onthe mid-portion of the leaf spring and prevent the axle from slidably moving thereon. Representing the distance between the eyelet portions 3 by the character (I), the angle ADB is made maximum and the radius of curvature is made minimum at the point which is apart from one of the eyelet portions 3 by the distance I- ls/2. Thus, the portion which extends from the center of the spring to both sides by the length ls/2 constitutes the supporting portion 20 having the same angle ADB and thesame radius of curvature. With this configuration, the supporting portion 20 provides a substantially flat lower surface for stably receiving the plate member 13. In the leaf spring designed as described above the supporting portion 20 does not provide spring action, so that it is practicalto assume in calculation that the load is applied on the point apart from the center of the eyelet portion by a distance I ls/2. Thus, the ratio (8) of the length of the flat portion to the spring length is reduced as compared with a spring which does not have the supporting portion 20,

but the ratio of the length (X,) to the effective spring length l ls/2 remains equivalent. Examples of spring having the supporting portions 20 are shown in Table 4. Each of the examples shown in Table 4 is the one in which the center of the supporting portion 20 is dismains the same irrespective of the distance from the center of the eyelet portion represent the supporting portions, and the ratio (8) is calculated with respect to the length obtained by subtracting the length (ls) of These embodiments are preferable to that of FIG. 7 having a recess 18 and a projection in that stress concentration can be avoided.

The invention has been described taking reference to the supporting portion from the total spring length (I). 5 embodiments each of which is so formed that the trans- TABLE 4 Example 5 (R Example 6 (R,.) Example 7 (R Load P (kg) 1,000 1.000 1.500

Spring Length I (mm) 665 665 666.8

Allowable Maximum Stress (f 50 (kg/mm) Spring Dimension hXh (mm) 100X5 8 x8 Angle at the Center Omax (deg) 143.8 162 149 Length of Flat Portion X (mm) 43.3 88.7 76.8

Ratio of Flat Portion to Total Length 12.6 (14.2) 25.7 (29) 23.0 (27.1) 5 r) Length of Support Portion 1s (mm) 80 80 Angle 0 Distance from the center of rad deg rad deg Distance from the center of rad deg eyelet x (mm) eyelet .r (mm) 0 0 0 0 0 0 0 0 43.3 0 0 0 0 30 0 0 50 0.10 5.7 0 0 50 0 0 88.7 0.64 36.7 0 0 76.8 0 0 100 0.79 45.3 0.62 35.5 99.3 0.785 45 150 1.23 70.5 1.27 72.8 150.4 1.31 75 200 1.60 91.7 1.74 99.7 197.5 1.75 100 250 2.01 2.22 127 252.4 2.27 300 2.46 140.9 2.77 159 283.4 2.60 149 305 2.51 143.8 2.82 162 333.4 do. do. 345 do. do. do. do.

FIG. 24 shows the relationship between the ratio (a) of the width to thickness of the blank and the angle (6) at the portion where the bending moment is largest as in FIG. 22 taking the ratio (8) of the length (2X;) of the flat portion to the length (I Is) as parameters. In FIG. 24, the examples 5-7 are shown by the points R through R In FIG. 24, there is also shown the relationship in springs in which the angle (0) is maximum at the point apart from the center of the eyelet portion of the spring by the distance lls/2 and having the dimension of the examples 14. The values corresponding to these examples are plotted in the drawing as shown by R, through R respectively. Corresponding points on FIG. 22 are shown by (X) in FIG. 24. This means that the corresponding points on FIG. 22 are displaced as shown by arrows in FIG. 24 due to the provision of the supporting portion. Further, the points as marked by A on the points of arrows drawn from the points R through R, show the angle (0) when the supporting portion 20 is not formed in each of the examples 5-7 so that the angle (0) is gradually increased toward the center of the spring.

In the embodiment shown in FIGS. 11 15, the recess 18 in FIG. 7 is omitted but the leaf spring 1 is provided with projections 21 at the opposite sides of the supporting portion 20 for engagement with cutouts 22 formed in the concave surface 16 of the intermediate member 15.

FIGS. 16 18 show a further modification in which the leaf spring is provided with inwardly recessed portions 23 on its outer surface for engagement with projections 24 formed on the concave surface 16.

verse center line of the blank assumes the shape of a part of a circle; however, it should be noted that the invention is not limited to such a shape but broad enough to include an elliptically curved shape, a parabolically curved shape or other curved shape of second degree.

FIG. 19 shows a cross-sectional view of an embodiment in which the transverse center line of the blank assumes a form of'parabola, the section being taken at the loading point where the angle (0) is largest, the angle (0) being defined by a line AD passing through one end of said transverse center line of the blank and the center of a circle defined by both ends and the center of the transverse center line, and another line BD passing through the other end of said transverse center line of the blank and the center of the aforementioned circle.

When the curved portion of the spring element has an elliptical or parabolic cross-section, both end portion of the transverse center line of a section are not parallel with each other even if the angle (0) exceeds Thus, this form is convenient for press forming. Further, each section can have a larger height which results in a higher section modulus. Thus, with this form, it is possible to increase the rate of change of the section modulus in each section enabling to design a short spanned leaf spring having a uniform stress distribution for a high load. With this form of design, it is possible to determine the maximum angle (0) within the range between 90 and 210.

In each of the preceding embodiments, the shape of section of the leaf spring element is synmetrical with 1 I respect toa line passing through the center C of the transverse center line of the section of blank and perpendicular to a linepassing through both ends A and B'of the transverse center line. Such a leaf spring may be used alone or, as in a usual automobile suspension device, a pair of leaf springs may be arranged in parallel relation. The present invention is not limited to such symmetrical forms, but in an application where a pair of leaf springs are arranged in parallel relation, the springs may be asymmetrical with respect to the said normal line passing through the point C. FIGS. 20 and 21 show an embodiment of such an asymmetrical form. In FIG. 20, there is shown a section of a spring where the load is applied or the angle is largest. In this case, the angle (0) is defined by a line AD passing through one end of the transverse center line of a sec- ;tion of blank and the center (D) of a circle defined by both ends A and B and the center C of the transverse center line and another line BD passing through the other-end B of the transverse center line and the center D of said circle. In an asymmetrical form, the angle (0) maybe determined within 90 to 180 at the loading point. When this type of asymmetrical spring is used in an automobile suspension device, it may be advisable that a pair of springs are arranged symmetrically as shown in FIG. 2l.'When each of the springs is so arranged that the portion of smaller radius of curvature is positioned at the outboard side, the suspension device can have a greater resistance to transverse oscillation.

As described above, according to the present invention, a leaf spring is made from a simple plate blank having a uniform thickness and. width throughout its length and has a flat-sectioned portion at each end for a length of 9 to 50 percent of effective spring length, the intermediate portion having a smoothly curved cross-section. The curved section has a greater section modulus as compared with the flat-sectioned portion so that, by selecting suitable shape of the curved section, it is possible to obtain substantially uniform stress distribution throughout the length of the spring. Thus, the fatigue life of the spring can be substantially increased.

With a design in which the angle as defined by a pair I of lines passing through both ends of the transverse center line of section of blank material and the center of 3. 99 2 sfinssila Sa d t endsa ad.tbsss of the transverse center line is largest at the loading point gradually decreasing towards the flat-sectioned portion, a more uniform stress distribution can be obtained. Said angle may be equal to or less than 180 when the curved portion has a part-circular section and may be up to 210 when the curved portion has a section of a curve of second degree such as a parabolic curve. Thus, it is possible to have a substantially large section modulus at the loading point. Therefore, a single spring element can withstand a substantial load. Further, if desired, it may be possible to use a plurality of such leaf spring elements to provide a laminated leaf spring.

What is claimed is:

1. A leaf spring comprising at least one leaf formed from a flat sheet of uniform width and uniform thickness and adapted. to be suspended at both ends thereof and loaded at its mid-portion, characterized in that an intermediate portion of the spring. including the midportion and extending over at least percent of the length of the spring is of arcuate cross section in planes perpendicular to the length of the. leaf and includes a convexed side being directed upwardly, the curvature of the arcuate crosssection diminishing towards and into flat end portions of the spring, the form of the cross section of the leaf at the intermediate portion of the leaf being such that the angle of the sector of the circle, which intersects the arcuate thickness-wise bisector line at the edges of the leaf and the width-wise bisector point of the line, covering the line between the edges of the leaf is not more than 210.

2. The leaf spring of claim .1, wherein the form of the arcuate cross-section of the leaf at the intermediate portion thereof is a portion of a curve of second degree and the said angle decreased gradually from the midportion towards the ends of the leaf so that the portion of arcuate cross-section smoothly connects with flat end portions of the spring.

3. The leaf spring of claim 2, wherein the total length of the flat end portions of the spring is not less than 9 percent of the total length of the spring.

4. The leaf spring of claim 3, wherein the mid-portion of the spring at which the load is applied is formed by plastic deformation with load receiving means on the convex side surface of the leaf.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,814,410 Dated June 4, 1974 Hiroshi FUKUI, Gakuji IWATSU and Junichi KATO Patent No.

Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title page, after "Foreign Application Priority Data" insert --Jan. 22, 1970 Japan ..6l50/70--.

Signed-and sealed this 8th day of October 1974.

(SEAL) Attest:

C. MARSHALL DANN McCOY M. GIBSON JR. Attesting Officer Commissioner of Patents USCOMM-DC 60876-P69 U.S, GOVERNMENT PRINTING OFFICE: I909 O--366-334.

F ORM PO-105O (10-69) 

1. A leaf spring comprising at least one leaf formed from a flat sheet of uniform width and uniform thickness and adapted to be suspended at both ends thereof and loaded at its mid-portion, characterized in that an intermediate portion of the spring including the mid-portion and extending over at least 50 percent of the length of the spring is of arcuate cross section in planes perpendicular to the length of the leaf and includes a convexed side being directed upwardly, the curvature of the arcuate cross section diminishing towards and into flat end portions of the spring, the form of the cross section of the leaf at the intermediate portion of the leaf being such that the angle of the sector of the circle, which intersects the arcuate thickness-wise bisector line at the edges of the leaf and the width-wise bisector point of the line, covering the line between the edges of the leaf is not more than 210*.
 2. The leaf spring of claim 1, wherein the form of the arcuate cross-section of the leaf at the intermediate portion thereof is a portion of a curve of second degree and the said angle decreased gradually from the mid-portion towards the ends of the leaf so that the portion of arcuate cross-section smoothly connects with flat end portions of the spring.
 3. The leaf spring of claim 2, wherein the total length of the flat end portions of the spring is not less than 9 percent of the total length of the spring.
 4. The leaf spring of claim 3, wherein the mid-portion of the spring at which the load is applied is formed by plastic deformation with load receiving means on the convex side surface of the leaf. 